Hi Lemurian,
Everything you say is correct, but not relevant to what I was dealing with in the post you quote!
In the early days of INS on airliners - like retrofitted dual-INS on B707s, or triple INS on B747s and DC-10s - the associated nav computers (one per INS) were not capable of AUTOMATIC position updating of any kind. What you could do, however, was to update the PPOS in the nav computer MANUALLY, if you were sure it was necessary. That is the scenario I was addressing.
Re automatic updating of PPOS in FMSs, DME/DME remained the most accurate method en-route until the incorporation of GPS around 1990. I say "en-route" because updating at the start of the take-off, and LLZ/DME on approach (where available), were (and are) more accurate.
One day, for one reason or another, Uncle Sam may have to turn GPS off. And he probably won't want to issue a NOTAM until after he's done it...